The present invention relates to a coherent integration enhancement method, a positioning method, a storage medium, a coherent integration enhancement circuit, a positioning circuit, and an electronic instrument.
The global positioning system (GPS) is widely known as a satellite positioning system, and is utilized for a car navigation system and the like. A GPS satellite signal is transmitted from each GPS satellite that orbits the earth. A GPS receiver calculates (locates) its present position based on the GPS satellite signals received from the GPS satellites.
The GPS receiver acquires and tracks the received GPS satellite signal, decodes a navigation message included in the GPS satellite signal, calculates the pseudo-range based on orbit information and time information of the GPS satellite included in the decoded navigation message, and calculates the present position. The GPS receiver acquires the GPS satellite signal (GPS satellite) by performing correlation calculations on the received signal and a code replica. The code replica is a signal that is pseudo-generated by the GPS receiver and simulates a coarse/acquisition (C/A) code (i.e., pseudo random noise (PRN) code) included in the acquisition target GPS satellite signal. Specifically, the GPS receiver performs a coherent process that calculates the correlation between the C/A code included in the received signal and the code replica using FFT calculations or the like, and an incoherent process that integrates the correlation values (coherent process results) to calculate an integrated correlation value. If the acquisition target GPS satellite signal is correct, the C/A code included in the GPS satellite signal coincides with the code replica (success). If the acquisition target GPS satellite signal is incorrect, the C/A code included in the GPS satellite signal does not coincide with the code replica (fail). Therefore, whether or not the GPS satellite signal has been successfully acquired can be determined using the calculated integrated correlation value. The GPS satellite signal can be acquired by performing correlation calculations on the received signal while changing the C/A code used as the code replica.
The following control is performed when performing correlation calculations. Specifically, the GPS receiver performs the coherent process and the incoherent process while changing the frequency of the code replica signal and the phase when calculating the correlation between the C/A code and the code replica. Specifically, the integrated correlation value becomes a maximum when the frequency of the code replica signal coincides with the frequency of the received signal (at an intermediate frequency) and the phase of the C/A code coincides with the phase of the code replica during correlation calculations (this phase is referred to as “code phase”).
In the GPS, the integration time of the coherent process is limited to 20 ms. This is because the GPS satellite signal is a signal obtained by subjecting the C/A code to binary phase shift keying (BPSK) modulates using the navigation message, and the transmission rate of the navigation message is 50 bps (bit per sec). Specifically, data obtained by performing the coherent process for an integration time equal to or longer than 20 ms may include phase inversion of the navigation message. If the coherent integration is performed across phase inversion, the amplitude of the correlation value is reversed across phase inversion so that cancellation occurs. As a result, the integrated value decreases. Therefore, when acquiring the GPS satellite signal, the coherent integration process is performed for an integration time equal to or less than 20 ms, and the incoherent integration process (integration of only the magnitude) is then performed on the correlation value.
As technology that enables the coherent integration process for an integration time equal to or longer than 20 ms without being affected by phase inversion of the navigation message, technology has been known which obtains the inversion pattern and the timing of the navigation message from a base station (i.e., external server) and performs correlation calculations in a state in which the polarity of the navigation message included in the received signal is made identical (see Japanese Patent No. 3787592, for example).
However, the method disclosed in Japanese Patent No. 3787592 requires the external server that detects the inversion pattern and the timing of the navigation message. Moreover, the GPS receiver must perform real-time data communication with the external server. The GPS receiver must also measure or acquire accurate time information in order to achieve synchronization with the external server.